The designing and manufacturing of silicon pressure sensors is a constantly developing art. The applications of small pressure sensors is constantly expanding because of their utility, due among other things to their small size, in many commercial applications. Among the commercial applications are in medical equipment such as blood pressure monitors, in air conditioning systems and toys. The use of small pressure sensor devices in the automotive industry is extensive, they are used in anti-lock braking devices, dynamatic suspension, crashbag or airbag activation, tire pressure monitoring, etc. There are many other industrial applications where small pressure transducers or sensors can be used, such as in chemical plants, food and beverage facilities, petrochemical plants, etc.
Given the small size of pressure sensors, the need for mass production while at the same time producing a high percentage of high quality sensors there is a persistent need to find a more efficient and effective way to manufacture them. Prior art includes: (a) conventional silicon pressure sensor fabricating method, the one most widely adopted by manufacturers which is based on a back-sided etching process; (b) a silicon fusion bonding method, which is a new technique used by some manufacturers and; (c) a single-sided process method different variations of which have been described in recently issued U.S. patents.
The conventional and most widely used manufacturing process of silicon diaphragm piezoresistive pressure sensors has a number of problems due in part the use of the back-sided etching processes. First, the processing steps are complicated, because a double-sided process is required which among other things requires a precise lining up of structures and work to be done on both sides of the substrate. Additionally, it is difficult to precisely control the thickness of the diaphragm made during fabrication which consequently affects the accuracy and sensitivity of the diaphragm. Also it is not easy to make the dimensions of the diaphragm of a sufficiently small size necessary for certain uses. Finally, it is necessary to bond the sensor chip to a glass or like base to form a hermetic reference chamber. All these problems are obstacles to improvements of the performance and to reduction of production costs of the current devices in use.
An example of the current types of manufactured silicon pressure sensors is set forth in FIG. 1a and b. FIG. 1a and b shows the general structure of a conventional silicon diaphragm piezoresistive pressure sensor. This silicon diaphragm piezoresistive pressure sensor includes a N-type silicon substrate 10 and a diaphragm 12 having P-type regions 14 on the surface. The substrate is etched from the back side to form the diaphragm.
There are two back-sided etching techniques widely used. One is a chemical etch-stop at a heavily boron-doped (about 10.sup.20 cm.sup.-3) layer. When a silicon epitaxial layer having a heavily boron-doped layer sandwiched in between an epitaxial layer and a substrate is etched by ethylene diamine pyrocatechol (EDP), the substrate is removed and the epitaxial layer is left. The other method is an electrochemical etch-stop process at a reverse-biased P-N junction. This technique combines an anodic passivation characteristic of silicon with a reverse-bias P-N junction to provide a large etching selectivity of P-type silicon over N-type silicon in anisotropic etchants such as a potassium hydroxide/water and hydrazine/water. When a positive voltage is applied to the N-type layer of a N/P epitaxial wafer immersed in hydroxide water or hydrazine/water etching can be stopped at the P-N junction.
Compared to the conventional silicon pressure sensor fabrication method, the present invention has the following advantages:
A. The present invention enables the sensor to be fabricated by what is called a single-sided processing method wherein all the processing steps are conducted solely on the upper side of the silicon substrate. Accordingly, the diaphragm and the reference pressure chamber are all formed by processing the substrate from one side. This greatly simplifies the manufacturing method compared to the conventional method which, and among other things, leads to a substantial reduction in production costs.
B. It is possible to form a diaphragm with a high degree of accuracy and avoid the problems caused by a lack of uniformity of thickness of the silicon substrate, a perennial problem with the present double-sided manufacturing method. Consequently, it is possible to fabricate diaphragms of relatively small and highly accurate dimensions in reference to a predetermined crystal plain dimension of the substrate. The ability to form a diaphragm of a predetermined and reduced thickness and dimension with high accuracy allows the production of sensors of much higher sensitivity and accuracy as compared to those made by current manufacturing methods.
C. Since the reference pressure chamber is formed within the bulk of the silicon substrate from one side, an absolute pressure sensor is formed with an air tight seal all which can be done by an integrated circuit fabrication techniques. The conventional manufacturing method has a persistent problem in providing for air tight bonding between the diaphragm and the base material a serious obstacle to effective and efficient mass production. Obviously with the technique as described herein with its simplified fabrication process the actual cost of manufacturing accurate and small pressure sensors can be substantially reduced.
D. The present invention enables the silicon pressure sensor to be formed by integrated circuit manufacturing techniques. This is possible because all of the processing steps are conducted by a one-sided processing method as described herein, consequently it is easy to design and treat the silicon pressure sensor itself as one element of an integrated circuit because the techniques of both manufacturing integrated circuits and the pressure sensor as described herein are substantially the same techniques. This allows for manufacture of combined pressure sensor and integrated circuits of predetermined signal processing characteristics with appropriate circuits, amplification and whatever addition devices are necessary for the use of the pressure sensor.
Another current fabrication technique used by some manufacturers is a silicon fusion bonding method. There are a number of problems with this technique: (A) The bonding of two silicon wafers together which is part of the process requires a high degree of cleanness and flatness of surfaces. In silicon fusion bonding the forces used are small and have short range so that surface roughness and dust particles can create major problems. Surface roughness and particles may prevent bonding or cause large "tent" areas several millimeters in diameter even if bonding is successful. In the manufacturing of absolute silicon pressure sensors such problems are totally unacceptable. (B) After the fusion bonding of two silicon wafers, other steps in this manufacturing process require the thinning to a predetermined thickness one of the wafers by conventional etching techniques. These conventional etching techniques create problems in the fabrication of a pressure sensor which can display a sufficiently high enough sensitivity for use in many applications.
Finally, recently issued U.S. patents have described single-sided processing methods which do take advantage of the single-sided process. However these patents described methods where the diaphragm and chamber are formed by orientation dependent etching through a small hole or holes on the upper side of a silicon substrate. The thickness and size of the diaphragm, as well as the dimensions of the chamber are determined by the length of time the actual etching process is allowed to proceed during the manufacturing process. How long the etching process is allowed to proceed is based on prior experience with etching speeds. However, the etching speed has a tendency to change during the process due to a number of factors, among them being the fact that the chamber which is being formed is becoming larger and effecting the etching process. As a result, it is impossible to make the diaphragm sufficiently uniform and thin. Nor is it possible to control or predetermine the actual size of the chamber.